New C₂-Fragment for Ruthenium-Catalyzed Synthesis of Indoles

We have demonstrated that anilines are cyclized with 1,2-dibromoethane in the presence of a ruthenium catalyst to give the corresponding indoles in moderate to good yields. The present reaction is an alternative synthetic approach for indoles from anilines and 1,2-dibromoethane as a new C₂-fragment. It is well known that indole moiety plays an important role as an intermediate for the synthesis of many pharmacologically and biologically active compounds. Thus, besides conventional popular routes, homogeneous transition metal-catalyzed synthetic methods have also been developed for the construction of indole framework because of the wide availability of substrates. In connection with this report, as the part of our ongoing studies on homogeneous ruthenium catalysis, we recently developed on the synthesis of indoles via a ruthenium-catalyzed alkanol group transfer from alkanolamines to N-atom of anilines and ring-opening of epoxides by anilines. Watanabe et al. have also reported a ruthenium-catalyzed intermolecular cyclization of anilines with ethylene glycols as C₂-fragment leading to indoles. In these regards, it was suggested that theses reactions proceed via a sequence involving an initial formation of 2-anilinoalkanols shown in and N-alkylation of anilines by 2-anilinoalkanols to form 1,2-dianilinoalkanes. These circumstances led us to seek a new C₂-fragment for such an intrinsic formation of 1,2-dianilinoalkanes